In automation technology, especially in process automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Serving for registering process variables are sensors, which are installed, for example, in fill level measurement devices, flow measuring devices, pressure- and temperature measuring devices, pH redox-potential measuring devices, conductivity measuring devices, etc., which register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, redox potential, and conductivity, respectively. Serving for influencing process variables are actuators, such as, for example, valves or pumps, via which, respectively, the flow of a liquid in a section of pipeline or the fill level in a container can be changed. Referred to as field devices are, in principle, all devices, which are applied near to the process and deliver, or process, process relevant information. In connection with the invention, the terminology, field devices, thus also includes remote I/Os, radio adapters, and, in general, all electronic components arranged at the field level. A great number of such field devices are manufactured and sold by the firm, Endress+Hauser.
Used for fill level measurement of fill substances in containers are contactless measuring methods. They offer the advantages of robustness and low-maintenance. A further advantage is their ability to measure the fill level virtually continuously, thus with a very high resolution. Applied in the field of continuous fill level measurement are primarily radar-based measuring methods. An established measuring principle, in such case, is the pulse travel time measuring principle, also known under the name, pulse radar. In such case, a microwave pulse is transmitted toward the fill substance and the travel time measured until receipt of the echo pulse. Using this measuring principle, pulse radar-based fill level measuring devices can be implemented comparatively without great circuit complexity. However, the resolution of this measuring device type is limited. Among the reasons for this is that the emitted microwave pulses cannot be infinitesimally short. Thus, the accuracy of measurement of the travel time is reduced and, so, also, the accuracy of measurement of the fill level.
To the extent that a more complex circuit technology can be tolerated, also FMCW (Frequency Modulated Continuous Wave) can be used as the measuring principle for radar-based fill level measurement. This principle enables a still higher resolution than possible with the pulse travel time measuring principle. The measuring principle of the FMCW-based radar distance measuring method rests on continuously transmitting a high-frequency microwave signal. In such case, the frequency of the signal lies in a fixed frequency band in the region of a standardized center frequency (f0). According to standard, here frequency bands in the 6 GHz band, the 26 GHz band, or the 79 GHz band are used. Characteristic for the FMCW method is that the transmitted frequency is not constant, but, instead, varies periodically within a frequency band. The frequency change can, in such case, be linear and have a sawtooth or triangular shape. A sinusoidal variation can, however, also be used, depending on application.
As in the case of pulse radar, there is also in the case of the FMCW-based fill level measuring method a special challenge in detecting the measurement signal free of doubt as to the correctness of the identification when disturbance signals are present. Thus, defective measured values can be generated, based on which the functional ability of the fill-level measuring device is degraded. A significant cause, in such case, is the receipt of disturbance echo signals, which arise not on the surface of the fill substance, but, instead, by reflection of the transmitted signal on disturbing bodies, such as stirrers or objects installed in the container.
In the interim, many technical approaches have been proposed, in order to identify, or filter out, these types of disturbance echo signals. Thus, known from Published International Application, WO 2012/139852 A1 is a method for calibration of FMCW-based fill-level measuring devices, in the case of which an unequivocal calibration-signal is generated by means of an oscillating reference reflector positioned between measuring device and fill substance.
An opportunity for preventing the registering of echo signals from disturbing bodies from the outset is provided by the application of a sounding tube or bypass tube. In such case, the sounding tube is placed in the container, for instance, vertically within the process space. In such case, atmosphere can flow in and out of the sounding tube in such a manner that the fill level of the fill substance within the sounding tube agrees with the fill level within the remaining process space.
In the case of a bypass tube, such is placed alongside the container, wherein it also, in this case, is connected with the process space in such a manner that also here the fill level in the bypass-pipe equals the fill level in the process space.
In the case of application of a sounding- or bypass tube, the fill-level measuring device is not, such as otherwise usual, arranged in such a manner that the antenna of the fill level measuring device is facing directly into the process space of the container, in which the fill substance is located. Rather, the fill-level measuring device is so placed on the upper end of the sounding tube that the antenna of the fill level measuring device transmits the microwave signal along the sounding tube toward the fill substance.
Fill level measuring devices, which are arranged on sounding tubes of round inner diameter, are calibrated, as a rule, using the tube inner diameter of the later used sounding tube. In the case of applications in the oil- and gas industry, calibration is frequently based on a reference inner diameter according to the standard DIN EN ISO 6708, for example, DN 100.
A problem occurs when the tube inner diameter of the sounding tube, on which the fill-level measuring device is placed in later use, does not exactly agree with the reference inner diameter of the calibration tube. This frequent case is disadvantageous to the extent that even a small difference between the inner diameters causes a significant measurement error in the fill level measurement and accordingly leads to a very inexact fill level measurement.